1. Technical Field
The present invention relates to pumps and, more particularly, high pressure liquid chromatography pumps.
2. Background of the Invention
In high pressure liquid chromatography (“HPLC”) systems, specialized pumps are used to produce fluid pressures in the range of 5000 to 6000 psi, or even more. Typically, the pumps are piston type pumps that employ pressure chambers having an upstream inlet with a non-return check valve and a downstream outlet with a non-return check valve. The pump piston is reciprocated by means of a rotating cam in engaging relation with one end of the piston. The other end of the piston has a plunger made of highly polished sapphire or ruby that has a diameter of 1 to 10 millimeters (0.04 to 0.4 inch) and is slidably held within the pump housing by suitable bearings and seals.
Since the pressure in HPLC systems must be carefully monitored to maintain a consistent flow rate, the cam is rotatively driven by way of an electronically-activated stepper or servo motor. As with any electrically-operated device, including stepper and servo motors, the motor torque requirements should be minimal so that smaller, less expensive motors may be used. One method of minimizing torque requirements is to use torque compensation. Examples of torque compensation are shown in Natwick et al U.S. Pat. No. 5,357,827 issued Oct. 25, 1994 and in Mossman et al. U.S. Pat. No. 6,267,559 issued Jul. 31, 2001. Therein, attempts were made to maintain and compensate motor load at a more constant level, since load pulsations require that a motor be used with torque capacity higher than maximum possible torque.
Several issues of concern are present in prior art constructions. First, the pumps often have overly complex configurations or constructions. Second, the stepper motors previously used had enough torque capacity to operate at maximum required torque but any excessive torque was not utilized for other purposes when torque peak was not present. Third, in the case of HPLC pumps, less that half of the rotational cycle requires high torque operation, while the rest of the cycle uses very little torque, if any.
While the prior art devices are generally sufficient for their intended function, other constructions may provide features that may be more desirable to the user.